Atomic clocks comprise a gaseous medium, often alkaline, a device for exciting the atoms of this gas such as a laser, capable of making them jump to higher energy states, and means for measuring a frequential signal emitted by the atoms on returning to the normal energy level, using the photons coming from the laser.
The frequency of the signal of the photons returned by the gas is defined by the formula ν=ΔE/h, where ν is the frequency, ΔE the difference between the energy levels and h Planck's constant, equal to 6.62×10−34 J/s.
It is known that this frequency is very stable and that it can thus serve as time reference unit. This is however no longer true when the Zeeman structure of the material is considered: the energy levels then appear as composed of sub-levels corresponding to slightly different states, which are distinguished by their angular momentum index mF, 0 for a reference state of the energy level and −1, −2, etc. or +1, +2, etc. for the others. This is illustrated by FIG. 1 in the case of the element 87Rb, the breakdown of the first two energy levels (of angular momentums F=1 and F=2) of which is shown.
The energy levels are sensitive to the ambient magnetic field. This sensitivity is low (of the second order) for the sub-level of angular momentum equal to 0, but much higher (of the first order) for the other sub-levels: the transitions made from or up to them produce photons, the frequency of which is variable and thus cannot serve as reference, and only the portion of the signal corresponding to the transition between the two sub-levels of zero angular momentum is exploited for the measurement, which adversely affects its quality. The reference frequency given by the clock is then fo=EO/h, where E0 is the energy difference between the sub-levels at mF=0 of the two states (F=1 and F=2 of the example of FIG. 1).
Alkaline gases have been preferred until now as measurement medium in atomic clocks since they generally comprise stable and excited states each provided with a sub-level with zero angular momentum that thus ensures a measurement at a stable resonance frequency. These bodies nevertheless have the drawback of being able to have several physical states at the ordinary operating conditions and to be chemically very reactive.
If it is possible to maintain the ambient magnetic field at a fixed value, all of the sub-levels are fixed and can contribute to the measurement. Several techniques for stabilising the ambient magnetic field have been developed and disclosed in certain publications, such as American patent US2007/0247241.